I can't speak for the HRX but I can say the HRR does a great job. I have the HRR216VYA and I'm very happy with it. I have thick Bermuda grass and the mower handles the grass very well with a nice looking cut.
A couple weeks ago I posted my experience with both the HRR and HRX mowers. in my case with damp, tall, thick fescue the HRR was inadeqate. I returned it and got the HRX. It has more power ,the grass did not stick much to the Nexite deck, the variable settings between mulching and bagging gave me a better, faster cut with less mower cleanup. A bonus was the HRX has a lower exhaust sound and less vibration.